III-Nitride semiconductors have significant applications for solid state lighting and lasers, power electronics, thermoelectricity, and solar cell applications. InGaN quantum wells (QWs) have been widely employed as an active region in nitride light-emitting diodes (LEDs) for solid state lighting application. The internal quantum efficiency in InGaN QWs LEDs is limited by: 1) high dislocation density leading to large non-radiative recombination rate, and 2) charge separation due to the existence of the electrostatic field in the QW leading to significant reduction of the radiative recombination rate. The detrimental effects become more severe for InGaN QWs, in particular as the emission wavelength is extended into the green or yellow spectral regimes Several approaches have been demonstrated to suppress the charge separation issue by employing novel QWs with improved electron-hole wave function overlap (Γe—hh) such as 1) nonpolar InGaN QWs, 2) staggered InGaN QW, 3) InGaN QW with δ-AlGaN layer, 4) type-II InGaN-based QW, 5) strain-compensated InGaN—AlGaN QW, and InGaN-delta-InN QW.
Another approach to enhance the radiative recombination rate and internal quantum efficiency of InGaN QWs active region is by employing surface-plasmon (SP) based LEDs. Since the InGaN QWs are coupled to surface plasmon mode at the interface of metallic film and semiconductor, the radiative recombination rate in the QWs can be enhanced due to the increased photon density of states near the surface plasmon frequency resulting from Purcell effect enhancement factor. The peak Purcell enhancement factor occurs at the surface plasmon frequency of a structure. Recent experiments have reported significant Purcell enhancement factor for InGaN/GaN QW by using a single Ag metallic layer, leading to an increase in internal quantum efficiency and radiative recombination rate. The use of a single metallic layer leads to strong enhancement near the surface plasmon frequencies, and the enhancement will reduce for frequencies further away from the surface plasmon resonant frequency. A problem exists, however, in that no enhancement is obtained for frequencies above the surface plasmon frequency of the single metallic layer structure.
Another recent approach, based on metallo-dielectric stacked structures, proposes “tuning” the surface plasmon frequency by using one or more metal layers each spaced apart by a dielectric layer. Tuning can be accomplished by changing the combination of dielectric and metallic material as well as the thickness of the dielectric spacer layer. However, the Purcell enhancement factor based on this approach becomes reduced for the frequency regimes away from the surface plasmon frequency of the particular metal in use. The metallo-dielectric approach also requires complex processing for hybrid deposition of both dielectric and metallic layers, since the deposition environment for the metal is different than the deposition environment of the dielectric.